For the production of urea granules various methods are known. In the past prilling was the predominant method, wherein a substantially anhydrous urea melt ( having a water content of no more than 0.1 to 0.3% by weight) is sprayed from the top of a prilling column in a rising stream of air of ambient temperature in which the droplets solidify to form so-called prills. These prills have a relatively small maximum diameter and are mechanically rather weak.
Urea granules having larger dimensions and better mechanical properties are presently being produced by the granulation of a substantially anhydrous urea melt or of an aqueous urea solution in a fluidized bed, for example as described in U.S. Pat. No. 4,219,589. In the process described therein, an aqueous urea solution having a urea concentration of 70-99.9% by weight, preferably 85-96% by weight, is sprayed in the form of very fine droplets having an average diameter of 20-120 .mu.m into a fluidized bed of urea particles at a temperature at which the water evaporates from the solution sprayed onto the particles, and urea solidifies on the particles to form granules having a desired size which my be 2.5 mm and more. As in this process rather large amounts of fly dust are formed, especially if the urea solution used as the starting material contains more than 5% by weight of water, in particular more than 10% by weight of water, preferably a crystallization retarder or granulation additive for the urea, in particular a water-soluble addition or condensation product of formaldehyde and urea, is added to the urea solution, whereby the formation of fly dust is considerably lower, or even suppressed. The result of the presence of the granulation additive is that the granules remain plastic as they are being formed, so that owing to rolling and/or impacts during the formation mechanically strong, smooth and round granules may be formed.
The resulting granules have a high crushing strength, a high impact resistance, and little tendency of forming fly dust through abrasion, and moreover do not cake together, not even upon prolonged storage, although urea exhibits a strong natural tendency of caking together.
Although the process of the cited U.S. patent specification resulted in large improvement in the properties of the urea granules, further improvement was still found to be necessary, especially with respect to the amount of fly dust of the final product and the dust formation during granulation.